Unicode is a character set standard which plans to codify all of the writing systems of the world,
plus many other symbols.

Unicode and ISO/IEC 10646 are coordinated standards that provide code points for characters in almost all modern character set standards,
covering more than 30 writing systems and hundreds of languages,
including all commercially-important modern languages.
All characters in the largest Chinese,
Japanese,
and Korean dictionaries are also encoded.
The standards will eventually cover almost all characters in more than 250 writing systems and thousands of languages.
Unicode 1.0 was released in October 1991,
and 4.0 in April 2003.

A Unicode character is an abstract entity.
It is not bound to any particular integer width,
especially not to the C language char.
Unicode is language-neutral and display-neutral: it does not encode the language of the text and it does not generally define fonts or other graphical layout details.
Unicode operates on characters and on text built from those characters.

Unicode defines characters like LATIN CAPITAL LETTER A or GREEK SMALL LETTER ALPHA and unique numbers for the characters,
in this case 0x0041 and 0x03B1,
respectively.
These unique numbers are called code points.

The Unicode standard prefers using hexadecimal notation for the code points.
If numbers like 0x0041 are unfamiliar to you,
take a peek at a later section,
"Hexadecimal Notation".
The Unicode standard uses the notation U+0041 LATIN CAPITAL LETTER A,
to give the hexadecimal code point and the normative name of the character.

Unicode also defines various properties for the characters,
like "uppercase" or "lowercase",
"decimal digit",
or "punctuation"; these properties are independent of the names of the characters.
Furthermore,
various operations on the characters like uppercasing,
lowercasing,
and collating (sorting) are defined.

A Unicode character consists either of a single code point,
or a base character (like LATIN CAPITAL LETTER A),
followed by one or more modifiers (like COMBINING ACUTE ACCENT).
This sequence of base character and modifiers is called a combining character sequence.

Whether to call these combining character sequences "characters" depends on your point of view.
If you are a programmer,
you probably would tend towards seeing each element in the sequences as one unit,
or "character".
The whole sequence could be seen as one "character",
however,
from the user's point of view,
since that's probably what it looks like in the context of the user's language.

With this "whole sequence" view of characters,
the total number of characters is open-ended.
But in the programmer's "one unit is one character" point of view,
the concept of "characters" is more deterministic.
In this document,
we take that second point of view: one "character" is one Unicode code point,
be it a base character or a combining character.

For some combinations,
there are precomposed characters.
LATIN CAPITAL LETTER A WITH ACUTE,
for example,
is defined as a single code point.
These precomposed characters are,
however,
only available for some combinations,
and are mainly meant to support round-trip conversions between Unicode and legacy standards (like the ISO 8859).
In the general case,
the composing method is more extensible.
To support conversion between different compositions of the characters,
various normalization forms to standardize representations are also defined.

Because of backward compatibility with legacy encodings,
the "a unique number for every character" idea breaks down a bit: instead,
there is "at least one number for every character".
The same character could be represented differently in several legacy encodings.
The converse is also not true: some code points do not have an assigned character.
Firstly,
there are unallocated code points within otherwise used blocks.
Secondly,
there are special Unicode control characters that do not represent true characters.

A common myth about Unicode is that it would be "16-bit",
that is,
Unicode is only represented as 0x10000 (or 65536) characters from 0x0000 to 0xFFFF.
This is untrue. Since Unicode 2.0 (July 1996),
Unicode has been defined all the way up to 21 bits (0x10FFFF),
and since Unicode 3.1 (March 2001),
characters have been defined beyond 0xFFFF.
The first 0x10000 characters are called the Plane 0,
or the Basic Multilingual Plane (BMP).
With Unicode 3.1,
17 (yes,
seventeen) planes in all were defined--but they are nowhere near full of defined characters,
yet.

Another myth is that the 256-character blocks have something to do with languages--that each block would define the characters used by a language or a set of languages.
This is also untrue. The division into blocks exists,
but it is almost completely accidental--an artifact of how the characters have been and still are allocated.
Instead,
there is a concept called scripts,
which is more useful: there is Latin script,
Greek script,
and so on.
Scripts usually span varied parts of several blocks.
For further information see Unicode::UCD.

The Unicode code points are just abstract numbers.
To input and output these abstract numbers,
the numbers must be encoded or serialised somehow.
Unicode defines several character encoding forms,
of which UTF-8 is perhaps the most popular.
UTF-8 is a variable length encoding that encodes Unicode characters as 1 to 6 bytes (only 4 with the currently defined characters).
Other encodings include UTF-16 and UTF-32 and their big- and little-endian variants (UTF-8 is byte-order independent) The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.

For more information about encodings--for instance,
to learn what surrogates and byte order marks (BOMs) are--see perlunicode.

Starting from Perl 5.6.0,
Perl has had the capacity to handle Unicode natively.
Perl 5.8.0,
however,
is the first recommended release for serious Unicode work.
The maintenance release 5.6.1 fixed many of the problems of the initial Unicode implementation,
but for example regular expressions still do not work with Unicode in 5.6.1.

Starting from Perl 5.8.0,
the use of use utf8 is needed only in much more restricted circumstances. In earlier releases the utf8 pragma was used to declare that operations in the current block or file would be Unicode-aware.
This model was found to be wrong,
or at least clumsy: the "Unicodeness" is now carried with the data,
instead of being attached to the operations.
Only one case remains where an explicit use utf8 is needed: if your Perl script itself is encoded in UTF-8,
you can use UTF-8 in your identifier names,
and in string and regular expression literals,
by saying use utf8.
This is not the default because scripts with legacy 8-bit data in them would break.
See utf8.

Perl supports both pre-5.6 strings of eight-bit native bytes,
and strings of Unicode characters.
The principle is that Perl tries to keep its data as eight-bit bytes for as long as possible,
but as soon as Unicodeness cannot be avoided,
the data is transparently upgraded to Unicode.

Internally,
Perl currently uses either whatever the native eight-bit character set of the platform (for example Latin-1) is,
defaulting to UTF-8,
to encode Unicode strings.
Specifically,
if all code points in the string are 0xFF or less,
Perl uses the native eight-bit character set.
Otherwise,
it uses UTF-8.

A user of Perl does not normally need to know nor care how Perl happens to encode its internal strings,
but it becomes relevant when outputting Unicode strings to a stream without a PerlIO layer -- one with the "default" encoding.
In such a case,
the raw bytes used internally (the native character set or UTF-8,
as appropriate for each string) will be used,
and a "Wide character" warning will be issued if those strings contain a character beyond 0x00FF.

For example,

perl -e 'print "\x{DF}\n", "\x{0100}\x{DF}\n"'

produces a fairly useless mixture of native bytes and UTF-8, as well as a warning:

Wide character in print at ...

To output UTF-8, use the :encoding or :utf8 output layer. Prepending

binmode(STDOUT, ":utf8");

to this sample program ensures that the output is completely UTF-8, and removes the program's warning.

You can enable automatic UTF-8-ification of your standard file handles, default open() layer, and @ARGV by using either the -C command line switch or the PERL_UNICODE environment variable, see perlrun for the documentation of the -C switch.

Note that this means that Perl expects other software to work, too: if Perl has been led to believe that STDIN should be UTF-8, but then STDIN coming in from another command is not UTF-8, Perl will complain about the malformed UTF-8.

All features that combine Unicode and I/O also require using the new PerlIO feature. Almost all Perl 5.8 platforms do use PerlIO, though: you can see whether yours is by running "perl -V" and looking for useperlio=define.

Perl 5.8.0 also supports Unicode on EBCDIC platforms. There, Unicode support is somewhat more complex to implement since additional conversions are needed at every step. Some problems remain, see perlebcdic for details.

In any case, the Unicode support on EBCDIC platforms is better than in the 5.6 series, which didn't work much at all for EBCDIC platform. On EBCDIC platforms, the internal Unicode encoding form is UTF-EBCDIC instead of UTF-8. The difference is that as UTF-8 is "ASCII-safe" in that ASCII characters encode to UTF-8 as-is, while UTF-EBCDIC is "EBCDIC-safe".

Naturally, ord() will do the reverse: it turns a character into a code point.

Note that \x.. (no {} and only two hexadecimal digits), \x{...}, and chr(...) for arguments less than 0x100 (decimal 256) generate an eight-bit character for backward compatibility with older Perls. For arguments of 0x100 or more, Unicode characters are always produced. If you want to force the production of Unicode characters regardless of the numeric value, use pack("U", ...) instead of \x.., \x{...}, or chr().

You can also use the charnames pragma to invoke characters by name in double-quoted strings:

use charnames ':full';
my $arabic_alef = "\N{ARABIC LETTER ALEF}";

And, as mentioned above, you can also pack() numbers into Unicode characters:

my $georgian_an = pack("U", 0x10a0);

Note that both \x{...} and \N{...} are compile-time string constants: you cannot use variables in them. if you want similar run-time functionality, use chr() and charnames::vianame().

If you want to force the result to Unicode characters, use the special "U0" prefix. It consumes no arguments but causes the following bytes to be interpreted as the UTF-8 encoding of Unicode characters:

my $chars = pack("U0W*", 0x80, 0x42);

Likewise, you can stop such UTF-8 interpretation by using the special "C0" prefix.

Handling Unicode is for the most part transparent: just use the strings as usual. Functions like index(), length(), and substr() will work on the Unicode characters; regular expressions will work on the Unicode characters (see perlunicode and perlretut).

Note that Perl considers combining character sequences to be separate characters, so for example

produces raw bytes that Perl happens to use to internally encode the Unicode string. Perl's internal encoding depends on the system as well as what characters happen to be in the string at the time. If any of the characters are at code points 0x100 or above, you will get a warning. To ensure that the output is explicitly rendered in the encoding you desire--and to avoid the warning--open the stream with the desired encoding. Some examples:

The matching of encoding names is loose: case does not matter, and many encodings have several aliases. Note that the :utf8 layer must always be specified exactly like that; it is not subject to the loose matching of encoding names. Also note that :utf8 is unsafe for input, because it accepts the data without validating that it is indeed valid UTF8.

Reading in a file that you know happens to be encoded in one of the Unicode or legacy encodings does not magically turn the data into Unicode in Perl's eyes. To do that, specify the appropriate layer when opening files

The :locale does not currently (as of Perl 5.8.0) work with open() and binmode(), only with the open pragma. The :utf8 and :encoding(...) methods do work with all of open(), binmode(), and the open pragma.

Similarly, you may use these I/O layers on output streams to automatically convert Unicode to the specified encoding when it is written to the stream. For example, the following snippet copies the contents of the file "text.jis" (encoded as ISO-2022-JP, aka JIS) to the file "text.utf8", encoded as UTF-8:

The naming of encodings, both by the open() and by the open pragma allows for flexible names: koi8-r and KOI8R will both be understood.

Common encodings recognized by ISO, MIME, IANA, and various other standardisation organisations are recognised; for a more detailed list see Encode::Supported.

read() reads characters and returns the number of characters. seek() and tell() operate on byte counts, as do sysread() and sysseek().

Notice that because of the default behaviour of not doing any conversion upon input if there is no default layer, it is easy to mistakenly write code that keeps on expanding a file by repeatedly encoding the data:

Sometimes you might want to display Perl scalars containing Unicode as simple ASCII (or EBCDIC) text. The following subroutine converts its argument so that Unicode characters with code points greater than 255 are displayed as \x{...}, control characters (like \n) are displayed as \x.., and the rest of the characters as themselves:

The bit complement operator ~ may produce surprising results if used on strings containing characters with ordinal values above 255. In such a case, the results are consistent with the internal encoding of the characters, but not with much else. So don't do that. Similarly for vec(): you will be operating on the internally-encoded bit patterns of the Unicode characters, not on the code point values, which is very probably not what you want.

Peeking At Perl's Internal Encoding

Normal users of Perl should never care how Perl encodes any particular Unicode string (because the normal ways to get at the contents of a string with Unicode--via input and output--should always be via explicitly-defined I/O layers). But if you must, there are two ways of looking behind the scenes.

One way of peeking inside the internal encoding of Unicode characters is to use unpack("C*", ... to get the bytes of whatever the string encoding happens to be, or unpack("U0..", ...) to get the bytes of the UTF-8 encoding:

The short answer is that by default Perl compares equivalence (eq, ne) based only on code points of the characters. In the above case, the answer is no (because 0x00C1 != 0x0041). But sometimes, any CAPITAL LETTER As should be considered equal, or even As of any case.

Character ranges in regular expression character classes (/[a-z]/) and in the tr/// (also known as y///) operator are not magically Unicode-aware. What this means is that [A-Za-z] will not magically start to mean "all alphabetic letters"; not that it does mean that even for 8-bit characters, you should be using /[[:alpha:]]/ in that case.

For specifying character classes like that in regular expressions, you can use the various Unicode properties--\pL, or perhaps \p{Alphabetic}, in this particular case. You can use Unicode code points as the end points of character ranges, but there is no magic associated with specifying a certain range. For further information--there are dozens of Unicode character classes--see perlunicode.

String-To-Number Conversions

Unicode does define several other decimal--and numeric--characters besides the familiar 0 to 9, such as the Arabic and Indic digits. Perl does not support string-to-number conversion for digits other than ASCII 0 to 9 (and ASCII a to f for hexadecimal).

Very probably not. Unless you are generating Unicode characters somehow, old behaviour should be preserved. About the only behaviour that has changed and which could start generating Unicode is the old behaviour of chr() where supplying an argument more than 255 produced a character modulo 255. chr(300), for example, was equal to chr(45) or "-" (in ASCII), now it is LATIN CAPITAL LETTER I WITH BREVE.

How Do I Make My Scripts Work With Unicode?

Very little work should be needed since nothing changes until you generate Unicode data. The most important thing is getting input as Unicode; for that, see the earlier I/O discussion.

How Do I Know Whether My String Is In Unicode?

You shouldn't have to care. But you may, because currently the semantics of the characters whose ordinals are in the range 128 to 255 is different depending on whether the string they are contained within is in Unicode or not. (See perlunicode.)

To determine if a string is in Unicode, use:

print utf8::is_utf8($string) ? 1 : 0, "\n";

But note that this doesn't mean that any of the characters in the string are necessary UTF-8 encoded, or that any of the characters have code points greater than 0xFF (255) or even 0x80 (128), or that the string has any characters at all. All the is_utf8() does is to return the value of the internal "utf8ness" flag attached to the $string. If the flag is off, the bytes in the scalar are interpreted as a single byte encoding. If the flag is on, the bytes in the scalar are interpreted as the (multi-byte, variable-length) UTF-8 encoded code points of the characters. Bytes added to an UTF-8 encoded string are automatically upgraded to UTF-8. If mixed non-UTF-8 and UTF-8 scalars are merged (double-quoted interpolation, explicit concatenation, and printf/sprintf parameter substitution), the result will be UTF-8 encoded as if copies of the byte strings were upgraded to UTF-8: for example,

$a = "ab\x80c";
$b = "\x{100}";
print "$a = $b\n";

the output string will be UTF-8-encoded ab\x80c = \x{100}\n, but $a will stay byte-encoded.

Sometimes you might really need to know the byte length of a string instead of the character length. For that use either the Encode::encode_utf8() function or the bytes pragma and the length() function:

If invalid, a Malformed UTF-8 character warning is produced. The "C0" means "process the string character per character". Without that, the unpack("U*", ...) would work in U0 mode (the default if the format string starts with U) and it would return the bytes making up the UTF-8 encoding of the target string, something that will always work.

How Do I Convert Binary Data Into a Particular Encoding, Or Vice Versa?

This probably isn't as useful as you might think. Normally, you shouldn't need to.

In one sense, what you are asking doesn't make much sense: encodings are for characters, and binary data are not "characters", so converting "data" into some encoding isn't meaningful unless you know in what character set and encoding the binary data is in, in which case it's not just binary data, now is it?

If you have a raw sequence of bytes that you know should be interpreted via a particular encoding, you can use Encode:

The call to from_to() changes the bytes in $data, but nothing material about the nature of the string has changed as far as Perl is concerned. Both before and after the call, the string $data contains just a bunch of 8-bit bytes. As far as Perl is concerned, the encoding of the string remains as "system-native 8-bit bytes".

In Perl, not very well. Avoid using locales through the locale pragma. Use only one or the other. But see perlrun for the description of the -C switch and its environment counterpart, $ENV{PERL_UNICODE} to see how to enable various Unicode features, for example by using locale settings.

The Unicode standard prefers using hexadecimal notation because that more clearly shows the division of Unicode into blocks of 256 characters. Hexadecimal is also simply shorter than decimal. You can use decimal notation, too, but learning to use hexadecimal just makes life easier with the Unicode standard. The U+HHHH notation uses hexadecimal, for example.

The 0x prefix means a hexadecimal number, the digits are 0-9 and a-f (or A-F, case doesn't matter). Each hexadecimal digit represents four bits, or half a byte. print 0x..., "\n" will show a hexadecimal number in decimal, and printf "%x\n", $decimal will show a decimal number in hexadecimal. If you have just the "hex digits" of a hexadecimal number, you can use the hex() function.

The Unicode support files live within the Perl installation in the directory

$Config{installprivlib}/unicore

in Perl 5.8.0 or newer, and

$Config{installprivlib}/unicode

in the Perl 5.6 series. (The renaming to lib/unicore was done to avoid naming conflicts with lib/Unicode in case-insensitive filesystems.) The main Unicode data file is UnicodeData.txt (or Unicode.301 in Perl 5.6.1.) You can find the $Config{installprivlib} by

perl "-V:installprivlib"

You can explore various information from the Unicode data files using the Unicode::UCD module.

If you cannot upgrade your Perl to 5.8.0 or later, you can still do some Unicode processing by using the modules Unicode::String, Unicode::Map8, and Unicode::Map, available from CPAN. If you have the GNU recode installed, you can also use the Perl front-end Convert::Recode for character conversions.

The following are fast conversions from ISO 8859-1 (Latin-1) bytes to UTF-8 bytes and back, the code works even with older Perl 5 versions.